Re: [PATCH 2/5] perf core: Set event's default overflow_handler

[Will Deacon <will.deacon@arm.com>]

On Wed, Mar 23, 2016 at 08:29:38PM +0100, Peter Zijlstra wrote:
> On Wed, Mar 23, 2016 at 06:13:49PM +0000, Will Deacon wrote:
> > On Wed, Mar 23, 2016 at 06:50:21PM +0100, Peter Zijlstra wrote:
> > > On Mon, Mar 14, 2016 at 09:59:42AM +0000, Wang Nan wrote:
> > > > +++ b/arch/arm/kernel/hw_breakpoint.c
> > > > @@ -631,7 +631,7 @@ int arch_validate_hwbkpt_settings(struct perf_event *bp)
> > > >  	info->address &= ~alignment_mask;
> > > >  	info->ctrl.len <<= offset;
> > > >  
> > > > -	if (!bp->overflow_handler) {
> > > > +	if (is_default_overflow_handler(bp)) {
> > > >  		/*
> > > >  		 * Mismatch breakpoints are required for single-stepping
> > > >  		 * breakpoints.
> > > > @@ -754,7 +754,7 @@ static void watchpoint_handler(unsigned long addr, unsigned int fsr,
> > > >  		 * mismatch breakpoint so we can single-step over the
> > > >  		 * watchpoint trigger.
> > > >  		 */
> > > > -		if (!wp->overflow_handler)
> > > > +		if (is_default_overflow_handler(wp))
> > > >  			enable_single_step(wp, instruction_pointer(regs));
> > > >  
> > > >  unlock:
> > > > diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
> > > > index b45c95d..4ef5373 100644
> > > > --- a/arch/arm64/kernel/hw_breakpoint.c
> > > > +++ b/arch/arm64/kernel/hw_breakpoint.c
> > > > @@ -616,7 +616,7 @@ static int breakpoint_handler(unsigned long unused, unsigned int esr,
> > > >  		perf_bp_event(bp, regs);
> > > >  
> > > >  		/* Do we need to handle the stepping? */
> > > > -		if (!bp->overflow_handler)
> > > > +		if (is_default_overflow_handler(bp))
> > > >  			step = 1;
> > > >  unlock:
> > > >  		rcu_read_unlock();
> > > > @@ -712,7 +712,7 @@ static int watchpoint_handler(unsigned long addr, unsigned int esr,
> > > >  		perf_bp_event(wp, regs);
> > > >  
> > > >  		/* Do we need to handle the stepping? */
> > > > -		if (!wp->overflow_handler)
> > > > +		if (is_default_overflow_handler(wp))
> > > >  			step = 1;
> > > >  
> > > >  unlock:
> > > 
> > > Will, why does it matter what the overflow handler is for this stuff?
> > 
> > Because ptrace registers an overflow handler for raising a SIGTRAP and
> > ptrace users (e.g. GDB) expect to handle the single-stepping themselves.
> > Perf, on the other hand, will livelock if the kernel doesn't do the
> > stepping.
> 
> Would it, perhaps, make sense to invert this test and check for
> ->overflow_handler == ptrace_hbptriggered instead? That way nobody gets
> surprise live-locks, endlessly triggering the same trap.

Not sure... I can imagine kgdb, for example, wanting to handle the stepping
itself. You also need to play clever tricks if you want to step through
LL/SC atomics, which the code here doesn't even try to handle (because
it involves disassembling the instructions and applying a bunch of
heuristics), so I imagine most debuggers wanting to take care of the step
themselves.

> But yes, this kinda blows.

Yup.

Will